1. Field of the Invention
This invention relates generally to pressure sensors and more particularly to a pressure sensor which is suitable to use in high precision digital air data computers for airborne measurement of altitude, airspeed, Mach number and related air vehicle parameters.
2. Description of the Prior Art
Present commercial and military aircraft require very precise measurement of pitot and static pressure for accurately determining airspeed, altitude, Mach number and related aerodynamic parameters. Most new aircraft utilize dedicated digital air data computers which include precision pressure sensors, a digital processor for computing required air data functions and output circuits for interfacing with other aircraft systems including cockpit displays. The pressure sensors for these computers are required to have a very high level of precision and resolution which exceeds the performance of state-of-the-art analog-to-digital converters. The sensors, therefore, must provide an output frequency or time period as a function of pressure which is compatible with providing a 20 to 24 bit digital input to the processor which is updated a minimum of 16 times per second. These sensors are required to maintain calibration for very long periods of time and to provide accurate performance in the normal environment of airborne electronic equipment. Calibration and thermal compensation are usually accomplished in the digital processor using coefficients which are stored in digital memory circuits packaged with the sensor.
There are a number of sensor configurations which meet the requirements for current aircraft applications. U.S. Pat. No. 3,456,508, also assigned to the present assignee, describes one such sensor which has been widely used in military, commercial, and general aviation systems. These devices typically require a high degree of manufacturing expertise and represent a significant portion of the cost, size and weight of modern digital air data computers. These sensors usually require complex calibration over a wide band of pressure and temperature to insure accuracy.
There is a continuing trend in reduction of both size and cost of digital electronics. There is also a trend toward combining air data functions with other computations as more powerful digital computer devices become available. These trends have made it increasingly important to develop a pressure sensor with required accuracy but with reduced size, weight and production cost. It is also desirable to reduce the calibration complexity and amount of computer time required for such sensor calibration.
The vibrating diaphragm pressure sensor described in U.S. Pat. No. 3,456,508 referenced above is one of the best pressure sensors presently available for air data applications. It uses the variation in natural frequency of a simple metal diaphragm as a function of applied pressure loading. The output frequency is related only to diaphragm characteristics and applied pressure load. Characteristics of the diaphragm drive electronics and frequency-to-digital conversion electronics do not have any significant effect on the sensor output accuracy. This sensor and other similar vibrating sensors must be mounted in a manner which isolates the sensor element from its surroundings at all frequencies within its operating range. There are also acoustic waves within the sensor cavity or attached tubing which contribute to thermal sensitivity and prevent these very accurate devices from being utilized for measuring differential pressure since acoustic forces on the vibrating element are a function of gas density.
The quartz diaphragm sensor of this invention is significantly smaller than present state-of-the-art air data pressure sensor devices. It provides a means for using the extremely stable mechanical properties and the piezoelectric properties of crystalline quartz to achieve a very stable, miniature, low cost pressure sensor. The variation of a piezoelectrically induced thickness mode oscillation in a cyrstalline quartz diaphragm as a function of pressure loading is used with suitable electronic circuits to provide a digital output. The use of a crystalline quartz diaphragm provides the extremely stable mechanical properties required for accuracy and calibration stability without the special heat treatment and material controls required for metal diaphragm sensors. Crystalline quartz is also more stable than fused quartz which is used in some state-of-the-art devices. The piezoelectric properties of crystalline quartz provide a simple means for exciting a thickness shear mode oscillation which does not have any measurable acoustic effects in the sensor cavity. This oscillation is completely isolated within the diaphragm itself and does not propagate to the sensor mounting. As with the vibrating diaphragm pressure sensor cited above, there are no mechanical linkages or electric circuits which contribute to errors or variations in the output frequency as a function of pressure. A reference oscillation on the same piece of quartz has been included to provide a difference frequency suitable for digital conversion and simultaneously provide compensations for variations in the output frequency as a function of temperature.
The sensor of this invention is simple in structure and suitable for manufacture using automated, multiple unit processes such as are used in the production of crystal oscillators for wrist watches and other applications requiring an accurate frequency reference with long term stability. As such it has lower production costs and lends itself to high volume production. It is also useable for many applications where cost and/or size have prevented use of a sensor of this accuracy class.